ASK THE EXPERTS: Array Sizing for Best Battery Charging

I am working with a customer in southern Oregon who has an existing stand-alone solar-electric system, and is considering adding an additional array and charge controller to help get the battery bank fully charged.

The existing system consists of a 0.5 kW 24-volt array, an older Trace C30A+ charge controller (from a previous system), an OutBack VFX3524 inverter/charger, a 24-volt, 850 Ah battery bank and an 8 kW Northern Lights generator. He is concerned that the array is not sufficient to fully charge the batteries and that the generator is running too often.

The existing array is partially shaded and I am not convinced that adding another array is a good solution. The owners are aware of what they can (and cannot) run on the batteries alone, and use the generator whenever they feel they would be overtaxing the batteries.

With a given amount of solar exposure and energy usage, what size array would be sufficient to charge the battery bank? Or would it be better to upgrade to a newer controller that uses MPPT technology, such as an OutBack FlexMax 60, instead?

Tom McDowell • via email

You have brought up several issues with your questions. First, the system you have described may best be labeled as a “generator-based system with PV assist,” because the generator, charging through the inverter, is the only source capable of fully charging the batteries. Some basic calculations will explain this. The ratio of battery capacity (in rated Ah) to the rate of charge or discharge (in amperes, or A) is called the C-rate. This ratio is used to quantify charge and discharge rates. For example, a common golf-cart battery has a capacity of 220 Ah. If a 22 A load is placed on the battery, it is being discharged at a C/10 rate (220 ÷ 22 = 10). If the battery is then recharged by a PV array producing 11 A, it’s being charged at a C/20 rate. A 1,000 Ah battery would need to be charged at 50 A to achieve the same C/20 rate.

A PV charge rate of C/20 or better is generally considered the minimum needed for good battery care. For the 850 Ah battery at 24 V, this would be 42 A (plus enough to meet the household loads)—or a PV array rated at more than 1,400 watts. This OutBack inverter/charger is capable of charging at 85 A continuously, according to published specifications. Assuming that the generator is properly set up to deliver its full AC current to the inverter at 120 VAC, this is a C/10 rate (850 Ah ÷ 85 A), which is adequate for proper charging and equalizing. The existing PV array can supply at best about 15 A to the batteries, less any energy used to run household loads while charging. This is about a C/56 charge rate (850 Ah ÷ 15 A)—too low to even overcome the batteries’ internal resistance, much less equalize the batteries.

In the earlier years of off-grid residential system design, PV modules were expensive and batteries were relatively inexpensive, and systems were designed accordingly. Extreme electrical energy efficiency was essential to live within the capacities of a home’s PV system. All of that has changed in recent years: Batteries have doubled in price and PV module prices have dropped to one-third of what they were 15 years ago. To add to this, homeowners now expect continuous performance from their systems—seldom do modern inverters go to sleep at night. While a 1 kW array was common and sufficient to supply a well-planned off-grid home’s needs 10 years ago, 2 kW and 3 kW arrays are more common today, and are affordable as well.

Today, it’s common, at least in relatively sunny climates, to design a system to provide only one to two days of autonomy. Arrays are usually sized to recover 100% of the average daily winter load in a single day. This approach tends to keep the batteries full throughout most of the year, leading to less generator run time, longer battery life, and better overall system performance. (For RE professionals reading this, see Christopher Freitas’ article “High-Capacity Battery Banks” in SolarPro 5.2 for more on this subject.)

This is the antithesis of your customer’s current system. I would suggest both adding to the array and upgrading to a modern MPPT charge controller and proper safety disconnects. Not only will a modern controller increase efficiency, but it will allow the array to be wired at higher nominal voltage: up to 60 V (up to 72 V in milder climates) with an OutBack FlexMax, and even higher with a MidNite Classic. This would allow locating the new array farther from the batteries, if there is a place with less shading.

Once that is done, keep the C30A on the old array, but keep an eye on it. The C30A has been out of production for 15 years, is quite unsophisticated, and had a reputation for failure (of the internal fuse holder and relay). It also lacks temperature compensation. Set it to regulate at around 29.5 V as a sort of trickle charger; the new controller will compensate. Lower its regulation voltage only if your customer has to add water more often than every few months (assuming that the batteries are not near the end of their life, when water loss increases). If and when the C30A fails, replace it with a simple, modern controller; the 15% to 20% increase in output provided by MPPT is hard to justify against its increased cost with a 500 W array.

Comments (2)

Hello, Anurag. Although Home Power typically doesn't cover combiner boxes with the level of detail necessary to design one, I was able to locate a helpful article from our other magazine, SolarPro. Sending it to you in PDF via email. Thanks very much for reading and best of luck.